The present invention relates to a method and an apparatus for manufacturing nitrogen-doped group II-VI compound semiconductor films.
Group II-VI compounds, which consist of group II elements such as zinc, cadmium, mercury and magnesium and group VI elements such as sulfur, selenium and tellurium, are semiconductors whose energy gaps range from the far infrared to the near ultraviolet and have now various uses, including an infrared sensor and a fluorescent material. Moreover, they are being investigated very actively for development as materials for blue and green color light emitting devices especially in recent years.
Of these group II-VI compounds, zinc sulfide (ZnS) and zinc solenoid (ZnSe) which have particularly large energy gaps are difficult of forming a p-type layer. To obtain a p-type layer of wide energy gap group II-VI compound semiconductors, the compounds need to be doped with lithium, sodium or a similar group I element or nitrogen, phosphorus or a similar group V element which serves as a p-type dopant In order that these dopants effectively act as acceptors, it is necessary that group I atoms substitute for group II atoms or group V atoms for group VI atoms. In a case where the group I element is used as the p-type dopant, however, these atoms are allowed to easily migrate to interstitial positions; hence, some of the doped atoms act as a donnor and a high mole concentration cannot be obtained. In case of using a group V element as a p-type dopant, if phosphorus or arsenic is doped in excess of a certain concentration, group V atoms are associated with themselves or with atomic vacancies to form carrier trapping centers and as high mole concentration cannot be obtained. It has been known, up to now, that in wide energy gap group II-VI compound semiconductors using such group I or V elements as dopants, a p-type layer of a practical hole concentration could be obtained only when using nitrogen, a group V element, as a dopant.
It had been difficult until several years ago, however, to dope nitrogen into the group II-VI compound semiconductor. Recently nitrogen can be doped by a method of exciting a nitrogen molecular gas in the form of a plasma, but this method is applicable only in the case of a molecular beam epitaxy (MBE) method which performs crystal growth under highly evacuated conditions; it is hard to apply the metal organic vapor phase epitaxy (MOVPE) method which conducts growth in a carrier gas as of hydrogen under low evacuated conditions.
An attempt has also been made to dope nitrogen in the MOVPE method using a nitrogen compound such as ammonia or trimethylamine. However, it is not become effective because the nitrogen compound has a high decomposition temperature and does not liberate nitrogen at such a low temperature as an ordinary growth temperature. Moreover, in case of using a compound in which nitrogen and hydrogen have a direct bond, such as ammonia, there is also presented a problem that a part of the nitrogen-hydrogen bonds is not cleaved and is incorporated intact in crystals.
The MOVPE method is relatively simple in the device configuration as compared with the MBE method, and hence is advantageous over the latter in terms of cost, maintenance, etc. It would be highly beneficial, therefore, if nitrogen could be doped in the MOVPE method. In case of doping nitrogen in the form of a plasma in the MBE method, a plasma generating device requires a high-frequency and a high voltage and high energy species in the plasma need to be removed so that crystals being grown are not irradiated with them. This inevitably leads to complexity in the device configuration. Thus, a simple method for doping the nitrogen in the MBE process would also be of great utility.
As described above, it is difficult in the MOVPE process to dope the nitrogen in the form of a plasma. The reason for this is that the nitrogen activated by being put in the state of a plasma collides with other gas molecules and becomes inactive again.